Feed-forward amplifiers have two cancellation loops: one for carrier cancellation and one for distortion cancellation. These cancellation loops naturally result in automatic gain and phase control of the amplifier that is “inside” the loops. Hence, power combing of multiple amplifiers is easy to accomplish with little or no effort.
Feed-forward amplifiers are being replaced with RF-input, digitally pre-distorted amplifiers. These amplifiers do not possess an automatic method for controlling gain or phase. Gain can be addressed with a minor amount of extra effort, but automatic phase control does not have an obvious, low-cost solution.
The prior art approaches this problem by matching the delay of the main amplifier path with the delay of a reference path. Once the delay of the two paths is matched, a variety of methods can be used to measure the relative insertion phase of the amplifier path against the reference path. One well-known technique would be to destructively combine a sample of each path, adjusting the amplifier path phase (and gain) to minimize the residual power. The power is typically read with some type of broadband detector, which can be built with a diode operating in the square law region. Variations on this detector include using correlators, log detectors, or phase detectors. The underlying principal is to match the delay of the amplifier path to the delay of the reference path so that broadband cancellation occurs, allowing the use of broadband detectors.
FIG. 1 shows a block diagram of such a prior-art amplifier system 100. Amplifier system 100 has an amplifier path and a reference path. The amplifier path includes variable amplitude adjuster 102, variable phase adjuster 104, and amplifier 106, while reference path includes delay element 108. In addition, amplifier system 100 has a feedback control loop comprising power detector 110 and microprocessor 112. The purpose of the reference path and the feedback control loop of amplifier system 100 is to ensure that, for a given input signal, the amplitude and phase of the output signal are at desired levels.
In operation, the amplitude and the phase of an input signal can be adjusted as needed by adjusters 102 and 104, respectively, before the signal is applied to amplifier 106. A portion of the input signal is tapped off at node 114 as a reference signal and applied to delay element 108, whose purpose is to ensure that the overall signal delay of the reference path matches the overall signal delay of the amplifier path. The delayed reference signal from delay element 108 is then applied to node 118, which also receives a portion of the output signal tapped off at node 116. Node 118 combines the two signals received from delay element 108 and node 116.
When the overall signal delay of the amplifier path identically matches the overall signal delay of the reference path, then adjusters 102 and 104 can be set such that the two signals applied to node 118 will be 180° out of phase and equal in amplitude for all signal frequencies. In that case, the interference between the two signals will be perfectly destructive for all frequencies, and power detector 110 should detect minimal power in the signal received from node 118. If the delay of the reference path were not designed (e.g., with delay element 108) to match the delay of the amplifier path, then the two signals input to node 118 would not be 180° out of phase for all signal frequencies, and the interference between the two signals would not be perfectly destructive for all frequencies. As used in the specification, the term “cancellation” refers to situations in which interference between two combined signals results in approximately perfect destruction, whether or not the two signals have exactly 180 degrees of phase difference and exactly equal magnitude and cancel each other completely. In such situations, the resulting combined signal will have minimal, if not zero, power. Those skilled in the art will appreciate that nodes 114 and 116 are typically implemented using couplers with appropriate scale factors that ensure that cancellation is substantially complete for nominal operations.
Microprocessor 112 monitors the detected power levels from detector 110 to control the adjustments made to the input signal by amplitude and phase adjusters 102 and 104 in order to minimize the detected power and thereby minimize the overall signal amplitude and phase differences between the amplifier and reference paths (also referred to herein as amplitude and phase offsets or amplitude and phase mismatches). Since delay element 108 nominally equalizes the overall signal delays between the amplifier and reference paths for all signal frequencies, then power detector 110 may be implemented using any of a wide range of types of power detectors, include either wide-band or narrow-band power detectors.
The cancellation technique of FIG. 1 works well when the delay of the amplifier path is short and/or building the reference path delay element is not too costly. If, however, the amplifier path delay is not short, then it can be expensive to realize a reference path delay that is broadband, relatively constant with time, operating power, and temperature, and easy to manufacture. The typical choices for an amplifier in the Universal Mobile Telecommunication Service (UMTS) band, for instance, would be a cavity filter, printed transmission lines, or coaxial cable. Each of these has its drawbacks in performance, and all of them are costly. For example, a rule of thumb might be to assume $1 per nsec of delay for a coaxial solution. A typical RF pre-distorted amplifier might have 80-85 nsec of delay, making this a costly solution.